thunderbolt: Export IOMMU based DMA protection support to userspace

Recent systems with Thunderbolt ports may support IOMMU natively. In
practice this means that Thunderbolt connected devices are placed behind
an IOMMU during the whole time it is connected (including during boot)
making Thunderbolt security levels redundant. This is called Kernel DMA
protection [1] by Microsoft.

Some of these systems still have Thunderbolt security level set to
"user" in order to support OS downgrade (the older version of the OS
might not support IOMMU based DMA protection so connecting a device
still relies on user approval).

Export this information to userspace by introducing a new sysfs
attribute (iommu_dma_protection). Based on it userspace tools can make
more accurate decision whether or not authorize the connected device.

In addition update Thunderbolt documentation regarding IOMMU based DMA
protection.

[1] https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderbolt

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Yehezkel Bernat <YehezkelShB@gmail.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/Documentation/admin-guide/thunderbolt.rst b/Documentation/admin-guide/thunderbolt.rst
index 35fccba6a9a6..898ad78f3cc7 100644
--- a/Documentation/admin-guide/thunderbolt.rst
+++ b/Documentation/admin-guide/thunderbolt.rst
@@ -133,6 +133,26 @@ If the user still wants to connect the device they can either approve
 the device without a key or write a new key and write 1 to the
 ``authorized`` file to get the new key stored on the device NVM.
 
+DMA protection utilizing IOMMU
+------------------------------
+Recent systems from 2018 and forward with Thunderbolt ports may natively
+support IOMMU. This means that Thunderbolt security is handled by an IOMMU
+so connected devices cannot access memory regions outside of what is
+allocated for them by drivers. When Linux is running on such system it
+automatically enables IOMMU if not enabled by the user already. These
+systems can be identified by reading ``1`` from
+``/sys/bus/thunderbolt/devices/domainX/iommu_dma_protection`` attribute.
+
+The driver does not do anything special in this case but because DMA
+protection is handled by the IOMMU, security levels (if set) are
+redundant. For this reason some systems ship with security level set to
+``none``. Other systems have security level set to ``user`` in order to
+support downgrade to older OS, so users who want to automatically
+authorize devices when IOMMU DMA protection is enabled can use the
+following ``udev`` rule::
+
+  ACTION=="add", SUBSYSTEM=="thunderbolt", ATTRS{iommu_dma_protection}=="1", ATTR{authorized}=="0", ATTR{authorized}="1"
+
 Upgrading NVM on Thunderbolt device or host
 -------------------------------------------
 Since most of the functionality is handled in firmware running on a